My cat is getting old =(

But shortly afterward in the following week, he's eating very little, his underside is... damp? from mouth to the anus for some unknown reason, and now he apparently has arthritis and hardly moves around anymore. He wasn't very active in the house for a few years before.

Probably urine. Goats do it naturally (pee on their undersides). If it can't move where does it pee other than the floor/itself?

My last cat died from some pimento using what's-it-called (some vehicle coolant).